Adenosine is a ligand which regulates cell signaling, which accounts for various physiological functions through specific adenosine receptors located in the cell membrane. Adenosine, an extracellular substance, acts as a neurotransmitter in a variety of physical systems, typically functioning to compensate for overactivity of certain organs and protect the body from the harmful effects of stress (Jacobson, K. A. et al., J. Med. Chem., 35, 407-422, 1992). These functions are based on a part of the negative feedback loop in which adenosine, formed through the dephosphorylation of endocellular or extracellular ATP (adenosine triphosphate), decreases the cellular energy and increases oxygen supply. Adenosine plays an important role in maintaining the homeostasis of organs such as the brain, the heart and the kidneys. For example, when externally administered to the brain, an adenosine agonist was proven to show neuroprotective effects and was also found to be involved in pain, recognition, exercise and sleep.
Pharmacological research and molecular cloning studies have thus far revealed two classes (P1 and P2) of adenosine receptors. In mediating signal transduction, P1 receptors are adapted for adenosine while P2 receptors are adapted for ATP, ADP, UTP and UDP. Four subtypes of P1 receptors have been identified. They can be divided into A1, A2 and A3 according to ligand affinity, distribution within the body, and functional pathway, and A2 further into A2A and A2B. These adenosine receptors are members of the G-protein-coupled receptor family. Pharmacological functions of adenosine A1, A2A and A2B receptors have been revealed using various selective ligands. As for the A3 receptor, it was first identified in 1992 (Zhou, Q. Y, et al., Proc. Natl. Acad. Sci., U.S.A., 89, 7432-7436, 1992) and its pathophysiological functions have been extensively studied.
Adenosine A1 and A2 receptor agonists, most derived from adenosine, have been intensively studied for use as hypotensive agents, therapeutics for mental illness and arrhythmia, lipid metabolism suppressant (therapeutics for diabetes) and neuroprotectives. On the other hand, their antagonists, derived from xanthine or in the form of two or more fused heterocyclic compounds, are developed as anti-asthmatics, anti-depressants, anti-arrhythmics, renal protectants, drugs for Parkinson's disease, and intelligence enhancers. Despite extensive study, only a few commercial products have been developed, including adenosine itself for the treatment of supraventricular tachycardia, and dipyridamole, the adenosine transfer inhibiting drug, which is used as a supplemental drug for warfarin in preventing blood coagulation after cardiotomy. The reason why little progress toward the commercialization of adenosine derivatives has been made is that because adenosine receptors are distributed throughout the body, and the activation thereof is accompanied by various pharmaceutical activities. In brief, there are no compounds that are able to activate only the adenosine receptors of a desired tissue.
The function of the adenosine A3 receptor, the most recently identified, remains unknown, in contrast to the A1 and A2 receptors, the functions of which are well known. Extensive research has been conducted to develop selective ligands of the adenosine A3 receptor. In this regard, three radiolabeled ligands [125I]ABA (N6-(4-amino-3-[125I]iodobenzyl)-adenosine), [125I]APNEA (N6-2-(4-amino-3-[125I]iodophenyl)-ethyl adenosine) and [125I]AB-MECA (N6-(4-amino-3-[125I]iodobenzyl)-adenosine-5′-N-methylcarboxamide) are currently used for the pharmacological study of adenosine A3 receptor. For example, it was found through research on the radiolabeled ligands that when expressed in Chinese Hamster Ovary (CHO) cells, the A3 receptor inhibited adenylyl cyclase, an enzyme that produces cAMP from ATP. Also, when activated by agonists, the A3 receptor was proven to mediate the activation of guanosine triphosphate-dependent phospholipase C, an enzyme which catalyzes the degradation of phosphatidyl inositol into inositol triphosphate and diacylglycerol (DAG) in the brain (Ramkumar, V. et al., J. Biol. Chem., 268, 168871-168890, 1993; Abbracchio, M. P. et al., Mol. Pharmacol., 48, 1038-1045, 1995). These findings indicate the possibility that there is a reaction pathway mediated by the A3 receptor in cerebral ischemia when it is activated. The reason is that this second messenger system acts as a reaction pathway leading to nerve injury in cerebral ischemia. Also, A3 receptor agonists are known to prevent cerebral diseases, such as epilepsy, and to protect the heart as well as inhibiting the release of TNF-α (tumor necrosis factor), an inflammation mediator, and the production of MIP-1α, interleukin-12 and interferon-γ, all acting as inflammation mediators. On the other hand, the inactivation of A3 adenosine receptor causes the release of inflammation factors, such as histamine, from mast cells, bronchoconstriction, and the apoptosis of immune cells. Accordingly, A3 adenosine antagonists have the possibility of being candidates as anti-inflammatory agents and anti-asthmatics. Therefore, compounds with pharmacological selectivity are believed to be drugs useful in the treatment of various diseases, including asthma, inflammation, cerebral ischemia, heart diseases, cancer, etc.
The nucleoside based compounds N6-(3-iodobenzyl)-5′-(N-methylcarbamoyl)-adenosine (IB-MECA) and N6-(3-iodobenzyl)-2-chloro-5′-(N-methylcarbamoyl)-adenosine (CI-IB-MECA) are representative human adenosine A3 agonists, and exhibit higher affinity and selectivity for the A3 adenosine receptor than for the A1 and A2 adenosine receptors. On the other hand, most potent and selective human A3 adenosine receptor antagonists possess non-purinergic heterocyclic skeleton compounds. However, nearly all of the non-purinergic heterocyclic human A3 adenosine antagonists are found to induce weak or ineffective activity through rat A3 adenosine receptor and thus were unsuitable for evaluation in small animal models, which is indispensable to the development of drugs for clinical application (Baraldi, P. G. et al., Curr. Med. Chem., 12, 1319-1329, 2005).
However, since A3 AR antagonists with nucleoside skeletons, in contrast to non-purinergic heterocyclic antagonists, exhibit high affinity and selectivity independent of species, the applicability thereof for animal test makes the nucleoside skeleton-based A3 AR antagonists preferred drug candidates. Accordingly, there is a need for the development of selective A3 antagonists based on nucleoside compounds.
Through various previous research, the present inventors discovered that nucleoside compounds must have an N-methylcarbamoyl group at position 5 of the sugar moiety and a base substituted with an arylamino group or alkylamino group at position 6 of the purine moiety for A3 adenosine receptor agonism, as in the representative materials IB-MECA and Cl-IB-MECA. Since, since the N-methylcarbamoyl group at position 5 of the sugar moiety forms a hydrogen bond to cause a conformational change essential for the agonism of the receptors (Kim, S-K. et al., J. Mol. Graph. Model., 25, 562-577, 2006), compounds devoid of an N-methylcarbamoyl at position 5 of the sugar moiety are thought to be strong candidates for A3 adenosine receptor antagonists.
Leading to the present invention, thorough and intensive research into A3 adenosine receptor ligands and pharmaceutical effects, resulted in the finding that specific adenosine derivatives selected on the basis of the structure-activity relationship thereof have high binding affinity and selectivity for A3 adenosine receptors compared to A1 or A2 adenosine receptors and can act as selective antagonists on A3 adenosine receptors, thus showing high therapeutic effects on inflammatory diseases.